An organic compound can take a wider variety of structures compared with an inorganic compound, and it is possible to synthesize a material having a variety of functions by appropriate molecular-design of an organic compound. Owing to these advantages, photo electronics and electronics which use a functional organic material have been attracting attention in recent years.
Examples of electronic devices in which organic compounds are used as functional organic materials include solar cells, light-emitting elements, organic transistors, and the like. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
It is said that light-emitting elements have a mechanism of light emission as follows: by applying voltage between a pair of electrodes with an electroluminescence (hereinafter also referred to as EL) layer interposed therebetween, electrons injected from a cathode and holes injected from an anode recombine with each other in an emission center of the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state; accordingly light is emitted. Singlet excitation and triplet excitation are known as excited states, and it is considered that light emission can be obtained through either of the excited states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like, in addition to the light-emitting layer.
EL materials for forming an EL layer are broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a low molecular material is often formed using an evaporation apparatus and a high molecular material is often formed using an ink-jet method or the like. A conventional evaporation apparatus, in which a substrate is mounted in a substrate holder, has a crucible (or an evaporation boat) containing an EL material, i.e., an evaporation material; a heater for heating the EL material in the crucible; and a shutter for preventing the EL material from being scattered during sublimation. The EL material heated with the heater is sublimed and formed over the substrate. In order to achieve uniform film formation, a deposition substrate needs to be rotated and the distance between the substrate and the crucible needs to be about 1 m even when the substrate has a size of 300 mm×360 mm.
In the case of considering manufacturing a full-color flat panel display using emission colors of red, green, and blue by the above method, a metal mask is provided between the substrate and an evaporation source so as to be in contact with the substrate, so that selective coloring can be achieved using this mask. However, this method does not provide very high-precision film formation and thus requires that the distance between pixels be designed to be large and that the width of a partition (a bank) formed of an insulator between pixels be large. Therefore, application of the method to a high-definition display device is difficult.
In addition, demands for more definition, higher aperture ratio, and higher reliability of a full-color flat panel display using emission colors of red, green, and blue have been increasing. Such demands are big objects in advancing higher definition (increase in the number of pixels) of a light-emitting device and miniaturization of each display pixel pitch with reduction in the size of the light-emitting device. At the same time, demands for improvement in productivity and cost reduction are increasing.
Thus, a method for forming an EL layer of a light-emitting element by laser thermal transfer has been proposed. Specifically, a transfer substrate is manufactured in which a metal layer including a high reflective layer and a low reflective layer (a light absorption layer) each for laser light is formed on a surface or rear surface of a supporting substrate and a material layer (an EL layer) for a light-emitting layer or the like is formed on the metal layer. After that, laser irradiation is performed from the rear surface side of the transfer substrate. The laser light is reflected by the high reflective layer and is absorbed by the low reflective layer and converted into heat, so that a material layer (an EL layer) over the low reflective layer is transferred (see Patent Document 1).
In Patent Document 1, heat which is converted from light in a light absorption layer is conducted not only in a film thickness direction but also in a film surface direction. When heat is conducted in a film surface direction, a heated region of the material layer is enlarged. As a result, the EL layer which is a transfer layer transferred to a deposition substrate is enlarged at the same time.